1. Field of the Invention
The invention relates to methods of polymerizing rosin.
2. Brief Description of the Prior Art
The term "polymerized rosin" refers to the resinous mixture obtained when rosin is treated under various conditions with strong inorganic acids, organic acids or acidic clays. The mixture comprises non-dimerizable resin acids such as dehydroabietic acid, resin acids containing some unsaturation which do not react and a number of different types of polymerized resin acids including dimerized resin acids. The mixture also contains a minor amount of esters, resin acid anhydrides and non-saponifiable substances. Polymerized rosin may be refined, usually by distillation, to remove some portion of the monomeric resin acids and other substances to produce a mixture containing a higher concentration of polymerized resin acids. This refined mixture always has a higher softening point than unrefined polymerized rosin and it is referred to as "dimerized rosin" in many textbooks.
Much prior art exists that bears upon the conversion of rosin to a more polymeric substance. Among the literature descriptions of prior art methods, U.S. Pat. Nos. 2,136,525; 2,108,982; 2,307,641; and 2,328,681 are examples; also the U.S. Pat. Nos. 2,515,218; 2,251,806; 2,532,120; and 4,105,462.
Previous methods described for polymerizing rosin have relied on the use of strong inorganic acids, such as sulfuric acid as the catalysts. In these procedures, the rosin is dissolved in an inert solvent during the polymerization reaction. A serious disadvantage of these methods is the formation of an acidic sludge that requires separation from the polymerized product.
Alternate methods have been described in which the catalysts are sulfonated organic polymers (U.S. Pat. No. 4,414,146), or a halogenated methanesulfonic acid (U.S. Pat. No. 4,339,377). The reaction is generally carried out in an inert organic solvent. Thus, the prior art for the polymerization of rosin includes both a catalyst and a solvent.
The present invention is an improvement over the prior art in that the polymerization is effected by formic acid which acts as both a solvent and a catalyst. This is unexpected in that two prior patents U.S. Pat. Nos. 2,375,618 and 2,492,146 teach against the operability of formic acid to function as such a catalyst. In fact, U.S. Pat. No. 2,375,618 specifically states that "heating rosin with an aliphatic carboxylic acid alone does not result in the preparation of a material of increased softening point". The polymerized rosin is recovered by a simple distillation of the solvent, formic acid. Furthermore, the latter can be effectively reused for subsequent cycles of polymerization. An additional advantage is that formic acid is an inexpensive and stable reagent.
The product formed by the formic acid procedure consists of 27-45% dimerized rosins, a yield comparable to those obtained by other methods. The dimerized rosins residue, left after distillation of the formic acid, can be directly esterified without prior purification of the dimer. The oxidation stability and physical properties of the dimerized rosin compare favorably with commercially available products, such as Sylvatac 95, produced by the Sylvachem Division of SCM Corporation.